The present invention relates to an optically-assisted magnetic recording head and an optically-assisted magnetic recording apparatus, and more particularly, to a novel optically-assisted magnetic recording head and an optically-assisted magnetic recording apparatus, capable of heating a magnetic recording medium by a light irradiation in order to magnetically record data to the medium with an extremely high density.
Magnetic recording apparatus for magnetically recording and reproducing information are under continuous development as large-capacity, high-speed and inexpensive information storage means. Especially, recent hard disc drive (HDD) has shown remarkable improvements. As proved on the product level, its recording density is over 15 Gbpsi (gigabits per square inch), internal data transfer rate is over 100 Mbps (megabits per second) and price is as low as several yens/MB (megabytes). The high recording density of HDD is due to a combination of improvements of a plurality of elements such as signal processing technique, servo control mechanisms, head, medium, HID, etc. Recently, however, it has become apparent that the thermal agitation of the medium disturbs the higher density of HDD.
The high density of magnetic recording can be attained by miniaturizing the recording cell (recording bit) size. However, as miniaturization of the recording cell progresses, the signal magnetic field intensity from the medium is reduced. So, to assure a predetermined signal-to-noise ratio (S/N ratio), it is indispensable to reduce the medium noise. The medium noise is caused mainly by a disordered magnetic transition. The magnitude of the disorder is proportional to a magnetization reversal unit of the medium.
The magnetic medium uses a thin film formed from polycrystalline magnetic particles (referred to as “multiparticle thin film” or “multiparticle medium” herein). In case a magnetic exchange interaction affects between magnetic particles, the magnetization reversal unit of the multiparticle thin film is composed of a plurality of exchange-coupled magnetic particles.
Heretofore, when a medium is to have the recording density of several hundreds Mbpsi to several Gbpsi, for example, noise reduction of the medium has been attained mainly by reducing the exchange interaction between the magnetic particles and making smaller the magnetization reversal unit. In the latest magnetic medium of 10 Gbpsi in recording density, the magnetization reversal unit is of only 2 or 3 magnetic particles. Thus, predictably, the magnetization reversal unit will be reduced to the size of only one magnetic particle in near future.
Therefore, to ensure a predetermined S/N ratio by further reducing the magnetization reversal unit, it is necessary to diminish the size of the magnetic particles. Taking the volume of a magnetic particle as V, a magnetic energy the particle has can be expressed as KuV where Ku is a magnetically anisotropic energy density the particle has. When V is made smaller for a lower medium noise, KuV becomes smaller with a result that the thermal energy near the room temperature will disturb information written in the medium, and reveals the problem of thermal agitation.
According to the analysis made by Sharrock et al., if the ratio between magnetic energy and thermal energy (kT, where k is Boltzman's constant and T is absolute temperature) of a particle, KuV/kT, is not 100 or so, it will impair the reliability of the record life. If reduction of the particle size is progressed for a lower medium noise with the anisotropy energy density Ku being maintained at (2to 3)×106 erg/cc of the CoCr group alloy conventionally used as a magnetic film in the recording medium, it is getting difficult to ensure a thermal agitation resistance.
Recently, magnetic film materials having a Ku value more than 107 erg/cc, such as CoPt, FePd, etc., have been attracting much attention. However, simply increasing the Ku value for compatibility between the small particle size and thermal agitation resistance will lead to another problem. The problem concerns the recording sensitivity. Specifically, as the Ku value of the magnetic film of a medium is increased, the recording coercive force Hc0 of the medium (Hc0=Ku/Isb; Isb is the net magnetization of the magnetic film of the medium) increases, and the necessary magnetic field for saturation recording increases proportionally to Hc0.
A recording magnetic field developed by a recording head and applied to the medium depends upon a current supplied to a recording coil as well as upon a recording magnetic pole material, magnetic pole shape, spacing, medium type, film thickness, etc. Since the tip of the recording magnetic pole is reduced in size as the recording density is higher, the magnetic field developed by the recording head is limited in intensity.
Even with a combination of a single-pole head that will develop a largest magnetic field and a vertical medium with a soft-magnetic backing, for example, its maximum recording field is only around 10 kOe (Oe: oersted). On the other hand, to ensure a sufficient thermal agitation resistance with a necessary particle size of about 5 nm for a future high-density, low-noise medium, it is necessary to use a magnetic film material having a Ku value of 107 erg/cc or more. In this case, however, since the magnetic field intensity necessary for recording to the medium at a temperature approximate to the room temperature is over 10 kOe, recording to the medium is disabled. Therefore, if the Ku value of the medium is simply increased, there will arise the problem of the recording to the medium being impossible.
As having been described in the foregoing, in the magnetic recording using the conventional multiparticle medium, noise reduction, thermal agitation resistance and higher recording density are in a trade-off relation with each other, which is an essential factor imposing a limit to the recording density.
An optically-assisted magnetic recording system will be able to overcome this problem. The optically-assisted magnetic recording system may be called thermally-assisted magnetic recording system, which may be categorized into the following two constructions. That is, the one construction employs a conventionally used multiparticle medium, the other construction employs a continuous magnetic medium that has been used in a magnetic optical recording system.
Preferably an optically-assisted magnetic recording system using a multiparticle medium uses magnetic particles as fine as sufficiently reducing noise and uses a recording layer exhibiting a high Ku value near the room temperature in order to ensure a thermal agitation resistance. In a medium having such a large Ku value, since the magnetic field intensity necessary for recording exceeds the intensity of a magnetic field developed by the recording head near a room temperature, recording is not possible. In contrast, in the optically-assisted magnetic recording system, locating a medium heating means such as light beams near the recording magnetic pole and locally heating the recording medium during recording to lower Hc0 of the heated portion of the medium below the magnetic field intensity from the recording head, and recording is effected.
Important points for realizing this basic concept are: recording should be completed by supplying a recording magnetic field during heating or before the heated medium cools down; only a limited area as small as the width of the recording pole should be selectively heated to prevent that adjacent tracks are undesirably heated and adjacent magnetic transition is destructed by thermal agitation.
In a mode using a multiparticle medium, in addition to thermal agitation of adjacent tracks, it is necessary to ensure that magnetic transition created in a track to be recorded does not give influences of thermal agitation to a downstream region which does not yet cool down sufficiently. However, it has the advantage that the recording density is determined by the particle size, and flux reversal speed is remarkably high.
On the other hand, a system using a continuous magnetic film, i.e. amorphous magnetic film, has shortcomings, not involved in multiparticle systems, that the recording density is determined by the thickness of the magnetic domain wall (10-20 nm) and, when accompanied with displacement of the domain wall, the speed of the domain wall displacement (tens of m/sec.) determines the data transfer speed. However, volume V of the magnetic particles can be regarded infinite, the problem of thermal agitation is out of problem. Also the system using a continuous magnetic film is equivalent to the multiparticle system in the respect of adjusting the coercive force of the medium near the room temperature higher than the head magnetic field and adjusting the coercive force of the medium of the heated portion lower than the head magnetic field.
A related art technique trying optically-assisted magnetic recording by using a magneto-optic medium as a continuous magnetic film is disclosed in, for example, Journal of the Magnetics Society of Japan vol. 23, 1999, No. 8, pp-1901-1906, the entire contents of this reference being incorporated herein by reference. Since this related art uses far-field light beams as the heat source for heating a medium and locating a recording pole and the beam source in confrontation with the medium, double-side recording is impossible and an evanescent light cannot be used.
Further, recording bit length is determined by the magnetic head, but since the recording track width is determined by the light spot size, the limit of the track width is restricted by the spot size of the far-field light. That is, even when combining a short-wavelength laser and a high NA lens, the limit of the track width is hundreds of nm, therefore, a higher recording density is hard to realize.
Moreover, since the precise alignment of the light-irradiated position and the recording field applying position is quite difficult in that related art, a higher recording density is hard to realize.
As explained above, the use of a multiparticle medium in an optically-assisted magnetic recording system involves various problems, namely, uncertainty of double-side recording, difficulty of employing an evanescent light, difficulty of realizing a precise alignment of the irradiating position and magnetic field applying position, difficulty of realization of a compact, light and inexpensive optically-assisted magnetic recording head, and particularly in a mode using an evanescent light
On the other hand, the use of a continuous magnetic film medium involves the problems: double-side recording being impossible, insufficient recording density due to impossibility of using an evanescent light, the recording density being limited by the misalignment of the light irradiating position and the magnetic field applying position.